1. Field of the Invention
The present invention relates to a bass enhancing method and circuit for enhancing a bass component of an audio signal, and to an audio reproducing system including the bass enhancing circuit and an electric audio transducer, such as a speaker or headphones.
2. Description of the Related Art
FIG. 8 shows a frequency characteristic of a compact speaker used, for example, in an active speaker having a power amplifier circuit. As shown in FIG. 8, the output level of the compact speaker decreases in the frequency range below a lowest reproduction frequency f0. The lowest reproduction frequency f0 (e.g., about 100 Hz) is typically within an audible frequency range.
Therefore, by enhancing a bass component at about or below the lowest reproduction frequency f0 in an audio signal, it is possible to produce a rich bass sound.
FIG. 9 and FIG. 10 illustrate an example of such a bass enhancing method.
In a bass enhancing circuit illustrated in FIG. 9, a low-pass filter (LPF) 71, for example, having a cutoff frequency slightly higher than f0 (100 Hz) extracts a bass component Slin of an input audio signal Sin (see FIG. 10), and a multiplier 72 multiplies the bass component Slin by a gain G calculated as described below. That is, the bass component Slin is amplified by the calculated gain G.
Additionally, in a manner to compensate for a phase delay in the low-pass filter 71 and the multiplier 72, a phase adjuster 73 adjusts the phase of the input audio signal Sin. Then, an adder 74 adds an audio signal Sinx obtained by the phase adjustment and a bass component Slg output from the multiplier 72. Thus, an output audio signal Sout in which only a bass component is enhanced is obtained.
The gain G is calculated by an absolute value detector 76, an envelope detector 77, and a gain calculator 78.
Specifically, the absolute value detector 76 detects the absolute value of the input audio signal Sin and obtains an absolute value ABS shown in FIG. 10. Subsequently, the envelope detector 77 detects an envelope of the absolute value ABS through an attack process and a release process and obtains an envelope value ENV indicated by a dashed line in FIG. 10.
Then, for example, if the input audio signal Sin is a digital signal and a full-bit or maximum level of the digital signal is 1.0 (0 dB), the gain calculator 78 uses the equation shown in FIG. 9 to calculate the gain G.
Thus, as shown in FIG. 10, the gain G gradually decreases during the attack time in which the envelope value ENV gradually increases, while the gain G gradually increases during the release time in which the envelope value ENV gradually decreases.
Then, as described above, the multiplier 72 multiplies the bass component Slin by the gain G, and the adder 74 adds the bass component Slg obtained by the multiplication to the audio signal Sinx obtained by the phase adjustment. Thus, as shown in the bottom part of FIG. 10, a bass component of the output audio signal Sout is enhanced.
If the input audio signal Sin is a digital signal, the bass enhancing circuit is configured as a digital processing circuit in which, for example, the low-pass filter 71 is realized as a finite impulse response (FIR) filter or an infinite impulse response (IIR) filter and the phase adjuster 73 is realized as a delay circuit or an all-pass filter (APF).
As an input to the absolute value detector 76, the audio signal Sinx obtained by phase adjustment or the output audio signal Sout may be used.
Japanese Unexamined Patent Application Publication No. 2000-286750 discloses a method of controlling the level of a digital signal, such as a digital audio signal. This method involves a process in which the absolute value of an input digital signal is converted to a gain value according to level control, the values of attack time and release time are added to the gain value, an envelope of the gain value is detected, and the input digital signal is multiplied by the envelope value.